Individual rods were physically isolated from the retina. Since these rods are isopotential and uncoupled from the network, measurements of light- and voltage-dependent changes in resistance at the rod membrane can be obtained. The studies show that the reversal potential for the light response is near 0 mV. The hyperpolarizing overshoot at the onset of the response persists even when the rod is hyperpolzrized with current, suggesting that the overshoot is a voltage-rather than light-dependent event. In the absence of sodium in the bathing medium, the overshoot reverses polarity near--75 mV and is associated with an increase in resistance, suggesting that it is mediated by a voltage-dependent inactivation of potassium. Strong outward rectification is blocked by TEA; inward rectification is blocked by cesium, suggesting two other voltage-dependent changes in potassium conductance. Under voltage clamp a region of negative resistance appears at the dark potential level when the rod has been previously depolarized. The possible mechanisms for negative resistance are discussed.
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